Ammonia is an intermediate compound which results from the decomposition of proteins. It is a common constituent in all domestic wastewater, as mammals eliminate most excess nitrogen via the urinary pathway in the form of Urea. This compound is quickly hydrolyzed after leaving the body, which releases ammonia. At the pH range which is normal for natural water, ammonia exists as the ammonium ion in wastewater, and as the primary reduced form of inorganic nitrogen in natural water.
Federal Water Pollution control statutes have, as their objective, the restoration and maintenance of the chemical, physical, and biological integrity of the Nation's water supply. This has been accomplished through the pursuit of two goals. The first was to reduce pollution of surface water, and the second was to prohibit the discharge of toxic compounds in toxic amounts. Ammonia has been found to be toxic to some forms of aquatic life at rather low concentrations. As a result, the U.S. Environmental Protection Agency, under the auspices of the Federal Water Pollution Control Act of 1972, and subsequent Amendments, is now placing significant emphasis on the control of ammonia in wastewater discharges. This control is achieved through the use of the National or State Pollution Discharge Elimination System permit program.
A common method of wastewater treatment is the activated sludge method. A flow chart for a typical activated sludge treatment process is shown in FIG. 1. The figure only shows the activated sludge process itself, various procedures which may precede or follow the process are not shown. This process involves maintaining a biomass in suspension. The biological mass rapidly absorbs the organic (carbonaceous) material in the wastewater which is then oxidized and used to accomplish cell growth.
The principal means of reducing the ammonia concentration in the wastewater using an activated sludge process is through the biological oxidation of ammonia to nitrate.
The biomass which is used in the activated sludge process generally contains two types of bacteria, heterotrophs and autotrophs. The heterotrophs absorb carbonaceous material and transform it into energy and cell growth. These bacteria have a high rate of growth. The autotrophs absorb ammonia and oxidize it into nitrates. These bacteria have lower growth rates and cell yield, and are more temperature and pH sensitive than the heterotrophs. Also, the nitrifying bacteria prefer an environment where suitable surface area is provided upon which to grow.
The biomass is mixed with incoming wastewater and is fed into a tank for aeration. Aeration replenishes the oxygen consumed by the process and provides mixing to keep the biomass in suspension. In these conventional systems, several hours of aeration is provided to accomplish the cell synthesis and the associated oxidation and aging of new growth. Generally, this time is about six hours. This aeration period is necessary to maintain the proper physiological state of the biomass in order to produce good separation of the biomass from the wastewater in the clarification process, resulting in a clear, high quality effluent.
The oxidation of ammonia to nitrate is a sequential, two step, biological process which involves two types of autotrophs. The process is outlined below. ##STR1##
The ammonia is oxidized to nitrite by Nitrosomas bacteria, and then is oxidized to nitrate by Nitrobacter bacteria. These two bacterial groups are autotrophic bacteria and use the ammonia as an energy source.
Heterotrophic bacteria, which use the carbon based material as a source of energy, have a high cell yield and undergo rapid growth. In contrast, the autotrophic bacteria have a low cell yield and slow growth. The autotrophs are also more temperature and pH sensitive than the heterotrophic bacteria. In addition, they are strictly aerobic and require the presence of several mg/l of oxygen to achieve optimum activity.
In a conventional activated sludge process, after the aeration treatment of the waste water with the biomass, the biological mass is separated from the flow by gravitational clarification. The net growth of cell mass must be removed from the system in order to maintain proper balance between the cell mass and incoming organic matter. The remainder of the biomass is returned to the influent end of the aeration process where it is mixed with incoming wastewater.
Although some lightly loaded wastewater plants achieve a satisfactory level of nitrification, many problems exist with the reliability of current nitrification technology in the conventional activated sludge process. If the wastewater flow is high in carbonaceous material, the growth of the heterotrophic bacteria is so much greater than the growth of the autotrophic bacteria, that the nitrifying bacteria are overgrown and "washed out" with the sludge wasting process. This causes a substantial impairment in the ability of the conventional activated sludge process to successfully achieve reliable nitrification in a one process system.
It has been known to provide submerged media throughout the entire length of the aeration tank to act as biomass support. However these systems suffer from the same overgrowth and wash out problems which are present in other types of conventional activated sludge system.
Thus, in most plants which have a heavy carbonaceous load, nitrification can not be undertaken effectively as an integral part of the normal activated sludge process. As a result, it is often necessary to have a separate nitrification process to treat the wastewater following the removal of the carbonaceous material. These separate nitrifying processes generally consist of a second sludge process employing aeration, clarification and return sludge, or a nitrifying filter. Nitrifying filters employ suitable surface area for the support of a nitrification biomass.
Each of these separate nitrification processes are expensive to construct and operate. The second sludge process is essentially equivalent to an activated sludge process in terms of capital investment, operation and maintenance costs. In addition, the biomass which is produced is a weak, poor quality floc which results in a poor quality effluent after clarification.
Newer nitrifying filters employ plastic media of various shapes as support for the nitrification bacteria. These filters are often 20 to 30 feet deep and generally require pumping of the wastewater flow. This type of design is subject to operational problems in cold weather, as the filters are subject to icing. Other forms of fixed media, such as rotating biological contractors have been employed in an attempt to achieve reliable nitrification. In all cases, the natural sloughing of the filters results in poor quality effluent that in general requires additional treatment before discharge. These disadvantages represent significant capital and operating costs.
A flow chart illustrating a secondary nitrification treatment system is illustrated in FIG. 2.
One older treatment system did provide adequate nitrification under certain conditions, but was not an activated sludge system. Specifically, trickling filters have been recognized for years as providing a means for achieving nitrification. The older, rock filled filters, if loaded lightly with carbonaceous material would achieve nitrification at the lower depths, especially during the warmer months. This occurred for two reasons. The primary reason was that the carbonaceous material was absorbed in the upper layers of the filter. This provided the nitrifying biological population the opportunity to exist in the lower depths of the filter without the interference of heterotrophic overgrowth. In addition, the natural ventilation system of the filter provided an oxygen rich atmosphere.